1. Field of the Invention
The present invention relates to surface acoustic wave filters for use, for example, as a band-pass filter, and more particularly, the present invention relates to an end-surface reflection type surface acoustic wave filter which utilizes an SH type surface acoustic wave, such as a BGS wave or a Love wave.
2. Description of the Related Art
A known end-surface reflection type surface acoustic wave device utilizing an SH type surface acoustic wave such as a BGS (Bleustein-Gulyaev-Shimizu) wave or a Love wave is a longitudinally coupled resonator type surface acoustic wave filter. In an end-surface reflection type surface acoustic wave device, a surface acoustic wave is reflected between two end-surfaces opposing each other. Accordingly, reflectors are not necessary and as a result, the surface acoustic wave device such as a surface acoustic wave filter can be miniaturized.
FIG. 14 is a perspective view of a conventional end-surface reflection type and longitudinally coupled resonator type surface acoustic wave filter. A longitudinally coupled resonator type surface acoustic wave filter 101 has a piezoelectric substrate 102. In the piezoelectric substrate 102 on the top surface thereof, a first groove 102a and a second groove 102b are formed so as to be parallel to each other and are spaced from each other by a predetermined distance. Between the grooves 102a and 102b, two interdigital transducers (IDTs) 103 and 104 are provided for constituting the longitudinally coupled resonator type surface acoustic wave filter.
In addition, side surfaces of the first and the second grooves 102a and 102b at the sides at which the IDTs 103 and 104 are located constitute reflection end-surfaces for reflecting an SH-type surface acoustic wave.
In the longitudinally coupled resonator type surface acoustic wave filter 101 described above, since reflectors are not necessary, miniaturization can be achieved. However, it has been required that an attenuation amount outside a pass band be further increased but this has not been possible previously.
In order to overcome the problems described above, preferred embodiments of the present invention provide an end-surface reflection type surface acoustic wave filter which greatly increases an attenuation amount outside the pass band and significantly improves the degree of selectivity.
In accordance with a first preferred embodiment of the present invention, an end-surface reflection type surface acoustic wave filter includes a piezoelectric substrate having first and second reflection end-surfaces disposed substantially parallel to each other and spaced from each other by a predetermined distance, a top surface, a bottom surface, first and second piezoelectric substrate portions extending from the bottom ends of the first and the second reflection end-surfaces to the outside, and first and second grooves or first and second recess portions open to the outside, the first and the second reflection end-surfaces and the first and the second piezoelectric substrate portions defining the first and the second grooves or the first and the second recess portions, respectively, a plurality of interdigital transducers disposed on the top surface of the piezoelectric substrate between the first and the second grooves or between the first and the second recess portions, and a resin-coating layer which is provided on the top surface of the piezoelectric substrate so as to cover at least the plurality of interdigital transducer and regions at which the first and the second grooves or the first and the second recess portions are disposed and which resin-coating layer includes a flexible resin. In the end-surface reflection type surface acoustic wave filter described above, the resin-coating layer partially intrudes into at least one of the first and the second grooves or at least one of the first and the second recess portions.
In accordance with a second preferred embodiment of the present invention, an end-surface reflection type surface acoustic wave filter includes a piezoelectric substrate having first and second reflection end-surfaces disposed substantially parallel to each other and spaced from each other by a predetermined distance, a top surface, a bottom surface, first and second piezoelectric substrate portions extending from the bottom ends of the first and the second reflection end-surfaces to the outside, and first and second grooves or first and second recess portions open to the outside, the first and the second reflection end-surfaces and the first and the second piezoelectric substrate portions defining the first and the second grooves or the first and the second recess portions, respectively, a plurality of interdigital transducers disposed on the top surface of the piezoelectric substrate between the first and the second grooves or between the first and the second recess portions, and a resin-coating layer which is provided on the top surface of the piezoelectric substrate so as to cover at least the plurality of interdigital transducers and regions at which the first and the second grooves or the first and the second recess portions are disposed and which resin-coating layer includes a flexible resin. In the end-surface reflection type surface acoustic wave filter described above, the resin-coating layer is arranged so as to cover approximately 20% or greater of an area of the top surface of the piezoelectric substrate where the plurality of interdigital transducers are disposed.
In the end-surface reflection type surface acoustic wave filter described above, the flexible resin may preferably have a Shore hardness of about 30 or less. When a resin having a Shore hardness of about 30 or less is used, while deterioration of insertion loss characteristics is suppressed, undesired spurious can be effectively prevented.
In the end-surface reflection type surface acoustic wave filter described above, as the flexible resin described above, a gel resin may be used. Consequently, while deterioration of insertion loss characteristics is suppressed, undesired spurious can be effectively prevented.
In addition, according to another preferred embodiment of the present invention, as the flexible resin described above, a resin having a density of approximately 1.2 g/cm3 or less or, more preferably, approximately 1.0 g/cm3 or less after being cured may be used. Consequently, while deterioration of insertion loss characteristics is suppressed, undesired spurious can be effectively presented.
In the end-surface reflection type surface acoustic wave filter described above, as the flexible resin described above, a resin having a Young""s modulus of about 1 MPa or less after being cured may be used. Consequently, while deterioration of insertion loss characteristics is suppressed, undesired spurious can be effectively prevented.
In the end-surface reflection type surface acoustic wave filter described above, as the flexible resin described above, a resin having a linear expansion coefficient of approximately 1.9xc3x9710xe2x88x924 (1/xc2x0 C.) or greater or, more preferably, approximately 2.3xc3x9710xe2x88x924 (1/xc2x0 C.) or greater after being cured may be used. Consequently, while deterioration of insertion loss characteristics is suppressed, undesired spurious can be effectively prevented.
As the flexible resin described above, a gel resin having a density of about 1.0 g/cm3 or less, a Young""s modulus of about 1 MPa or less, and a linear expansion coefficient of about 2.3xc3x9710xe2x88x924 (1/xc2x0 C.) or greater after being cured is more preferably used.
As the flexible resin described above, for example, a silicone rubber, an epoxy resin, or a urethane rubber may be used, and more preferably, a silicon rubber is used.
In accordance with a third preferred embodiment of the present invention, an end-surface reflection type surface acoustic wave filter includes a piezoelectric substrate having first and second reflection end-surfaces disposed substantially parallel to each other and spaced from each other by a predetermined distance therebetween, a top surface, a bottom surface, first and second piezoelectric substrate portions extending from the bottom ends of the first and the second reflection end-surfaces to the outside, and first and second grooves or first and second recess portions open to the outside, the first and the second reflection end-surfaces and the first and the second piezoelectric substrate portions defining the first and the second grooves or the first and the second recess portions, respectively, a plurality of interdigital transducers disposed on the top surface of the piezoelectric substrate between the first and the second grooves or between the first and the second recess portions, and a layer which includes SiO2 and is arranged so as to cover the plurality of interdigital transducers.
In addition, in the end-surface reflection surface acoustic wave filter in accordance with the third preferred embodiment of the present invention, when a wavelength of a surface acoustic wave which is to be used is represented by xcex, and an electrode thickness of the interdigital transducer is represented by H, H/xcexxe2x89xa60.06 is preferably satisfied, and H/xcexxe2x89xa60.045 is more preferably satisfied. In the case described above, deterioration of insertion loss characteristics can be effectively prevented.
In the end-surface reflection type surface acoustic wave filters in accordance with the first to the third preferred embodiments of the present invention, the grooves or the recess portions each preferably have a depth that is substantially equal to or larger than a wavelength of an SH type surface acoustic wave which is to be used. Propagation of an SH type surface acoustic wave is primarily performed at a depth that is substantially equivalent to or smaller than the wavelength thereof from the surface of the piezoelectric substrate. Accordingly, when each of the grooves or the recess portions has a depth in accordance the range described above, the SH type surface acoustic wave which is to be used is reliably reflected on the end-surfaces, and hence a surface acoustic wave filter having superior properties can be provided.
The structures of the end-surface reflection type surface acoustic wave filters in accordance with the first to the third preferred embodiments of the present invention are not specifically limited, and it may be a ladder type filter including an end-surface reflection resonator, a longitudinally coupled resonator filter, or a transversely coupled resonator filter.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.